Given a linear operator L^ acting on x∈[a,b],
its fundamental solutionG(x,x′) is defined as the response
of L^ to a Dirac delta functionδ(x−x′) for x∈]a,b[:
L^{G(x,x′)}=Aδ(x−x′)
Where A is a constant, usually 1.
Fundamental solutions are often called Green’s functions,
but are distinct from the (somewhat related)
Green’s functions
in many-body quantum theory.
Note that the definition of G(x,x′) generalizes that of
the impulse response.
And likewise, due to the superposition principle,
once G is known, L^’s response u(x) to
any forcing function f(x) can easily be found as follows:
L^{u(x)}=f(x)⟹u(x)=A1∫abf(x′)G(x,x′)dx′
L^ only acts on x, so x′∈]a,b[ is simply a parameter,
meaning we are free to multiply the definition of G
by the constant f(x′) on both sides,
and exploit L^’s linearity:
Af(x′)δ(x−x′)=f(x′)L^{G(x,x′)}=L^{f(x′)G(x,x′)}
We then integrate both sides over x′ in the interval [a,b],
allowing us to consume δ(x−x′).
Note that ∫dx′ commutes with L^ acting on x:
By definition, L^’s response u(x) to f(x)
satisfies L^{u(x)}=f(x), recognizable here.
While the impulse response is typically used for initial value problems,
the fundamental solution G is used for boundary value problems.
Suppose those boundary conditions are homogeneous,
i.e. u(x) or one of its derivatives is zero at the boundaries.
Then:
This holds for all x′, and analogously for the other boundary x=b.
In other words, the boundary conditions are built into G.
What if the boundary conditions are inhomogeneous?
No problem: thanks to the linearity of L^,
those conditions can be given to the homogeneous solution uh(x),
where L^{uh(x)}=0,
such that the inhomogeneous solution ui(x)=u(x)−uh(x)
has homogeneous boundaries again,
so we can use G as usual to find ui(x), and then just add uh(x).
If L^ is self-adjoint
(see e.g. Sturm-Liouville theory),
then the fundamental solution G(x,x′)
has the following reciprocity boundary condition:
G(x,x′)=G∗(x′,x)
Consider two parameters x1′ and x2′.
The self-adjointness of L^ means that: